In a receiver of a communications system such as an Ethernet communications system, an eye pattern can be obtained that is a superposition of all possible realizations of the received signal as viewed within a particular signaling interval. The width of the eye opening defines the time interval over which the received signal can be sampled without imposition of errors such as errors caused by inter-symbol interference. Additional parameters associated with an eye pattern such as the slope of the pattern can be a measure of receiver sensitivity, while the vertical eye opening can define a noise margin for the receiver.
Typically, on-line measures of vertical eye opening can be obtained with limited additional hardware on a receiver. However, on-line horizontal eye measurements require an additional phase interpolator, increasing the cost, complexity, area, and power consumption of the receiver due to this costly analog circuit.